This invention relates to quick-acting electron-optical lenses.
There are many items of electron-beam equipment which employ electron guns of small diameter for technical and scientific purposes, examples being cathode-ray tubes for television or other reproducing purposes, electron microscopes, and materials-processing equipment.
To enable the electron beam emitted by the cathode to be focussed, complicated electron-optical lenses and diaphragm systems are provided in some cases. There are applications in which the focus has to be re-adjusted after the electron beam has been deflected, e.g. in cathode ray tubes, and there are cases in materials processing in which, with the electron beam stationary or in course of deflection, the focus is deliberately altered to produce a desired processing effect.
To enable this alteration in focus to be performed, a further focussing lens, also known as a dynamic focussing lens, is often provided in addition to the main lens, also termed the static lens, the static main lens being responsible for the overall foscussing and the re-adjustment or control of the position of the focus being being performed dynamically via the auxiliary lens. The lenses in question are generally in the form of magnetic coils.
In known electron-beam apparatus which employs an auxiliary lens for the dynamic focussing of the beam, the said lens is normally positioned between the beam generating system and the main lens. In materials processing using electron beams it has been found that there are instances where the focus has to be adjusted by a specific amount in a very short space of time. This of course is also a problem which occurs in types of reproduction apparatus which employ an electron beam, in cases where a high picture repetition frequency is required and the electron beam must therefore be deflected quickly and thus re-focussed quickly. To achieve the fast change in magnetic field which is required to produce a given change in the position of the focus, the dynamic auxiliary coil generally requires very high levels of energising power. In materials processing there are instances where the focus of the electron beam has to be adjusted in a time so short that the energising power required for the focussing coil is 1 kw or more. Since there are problems is handling such power levels, that is to say in changing the magnetic induction within the coil at the speed required, it is an object of the present invention to provide an improved dynamic focussing coil for electron-beam apparatus which requires a substantially lower power level.